16-state quadrature amplitude modulation (16-QAM) is widely used in the field of radio communications because of its high spectral effectiveness (compared with the commonly used techniques of phase modulation and of frequency modulation), and because it is relatively simple to implement (implementing the modulator/demodulator, processing the signal, etc.).
Furthermore, in the context of radio transmission, the cost of leasing transmission channels is directly related to the transmitted spectrum width. That is why the channel encoding technique chosen for this type of link must have a high rate.
Finally, error-correcting encoding must not only have performance levels that are acceptable from the system point of view, but must also be as simple as possible to implement, in particular as regards the decoder.
In the context of the present invention, the data transmitted from a transmitter to a receiver is integrated into a framed structure having fixed redundancy. In which case, block coded modulation is preferable to trellis coded modulation (TCM).
It is known that, for such transmission, it is possible to use modulation and high-rate convolutional encoding that are separated from each other. In the case of a framed structure having a fixed rate (N redundancy bits being added to every K message information bits to be transmitted), 16-QAM modulation associated with a block code like the Reed-Solomon code with firm decision is a natural choice.
Unfortunately, the choice of convolutional encoding either separate from or associated with modulation (TCM encoding) implies a rate that is necessarily limited. In addition, such a choice is not compatible with the choice of a framed structure having a fixed rate. Furthermore, the corresponding decoder associated with a convolutional code (Viterbi algorithm) is relatively complex.
For modulation associating 16-QAM mapping with Reed-Solomon coding, it is necessary to use a complex decoder. Furthermore, that solution does not enable decoding to take place under flexible-decision conditions.
Moreover, when coherent demodulation is performed by a carrier-recovery system enabling the phase and the frequency of the signal to be synchronized on reception, there still remain phase ambiguities that must be removed after such synchronization. For example, all QPSK and n-QAM constellations (two-dimensional constellations) have three phase ambiguities (+.pi./2, -.pi./2, and .pi.) which must be removed. The encoding scheme is said to be "transparent to phase ambiguities" if it enables the sequence to be retrieved on reception as transmitted regardless of the phase ambiguity imparted on reception by carrier-recovery.